Zombies

Zombies (or ”Walkers”, "Roamers" or "Biters" as referred to in the series) are dead people that have reanimated by an undisclosed pandemic.

Since already dead, the only way to actually kill a zombie is to cause severe trauma to the brain, killing off all brain signals. The way in which the zombies are able to come back has never been fully explained, but they retain some functionality such as the ability to move, attack and react to the presence of sounds.

They are extremely dangerous, often underesimated foes. When alone they can often be dispatched relatively easily, but many a person in The Walking Dead universe has either been injured or killed by becoming complacement with Walkers. They are most dangerous when travelling in packs, where their sheer numbers often overwhelm survivors.

Death and Reanimation

Once a person dies in The Walking Dead, their original spark of life is put out entirely. Their body, or what is left of it, then returns as a Walker.

The time in which the body returns to 'life' is varied. Some people come back as walkers almost instantly, whereas others have been known to take eight hours to fully reanimate.

Once returned, the Walker is devoid of all human traits. They only live to feed, and are able to detect living humans and will do anything to get to the fresh human flesh.

The Virus

It is believed originally that all Walkers carry a highly contagious virus which can be transmitted either by bite, or in some cases, a scratch.

This is revealed eventually to be untrue - everyone who is alive in The Walking Dead is carrying the virus, meaning that they will always turn into a Walker upon death.

The threat of a bite or scratch from a Walker is still an extremely high threat despite this fact. The bite in particular seems to accelerate the virus' spread throughout a human host, ultimately causing them to lose functionality and, eventually, die.

As a result of this many characters in the universe have taken to drastic actions to stop the spread of the virus, sacrificing others to save their own lives or chopping off affected limbs to hopefully stop the spread of the virus throughout their system.

It is unknown exactly what caused the virus. The series is based not around the discovery of a cure, but generally is focused on characters and the need to survive at all costs.

It appears to only affect humans. Animals that die do not return as Walkers.

The Pandemic

In the beginning, the zombie pandemic started out small. It is not know exactly what caused it, but we do know that the rapid spread of the disease was caused by the world being unprepared for such a catastrophe.

In the beginning, the government tried unsucessfully to save the lives of thousands of people by advising they move into cities. This was ultimately the catalyst for the creation of thousands of Walkers; the proximity of so many people together meant the disease was spread extremely rapidly, leading to the cities becoming one of the most dangerous places in The Walking Dead universe.

The infection also spread rapidly through other locations in the universe, meaning practically nowhere is safe.

Effects on Society

The constant threat of Walker attack means the world changes dramatically in The Walking Dead universe. What was once a world of normalcy descends into chaos and terror, leading to those few who have survived doing absolutely anything they can to survive.

In fact, one of the biggest dangers of the Walking Dead universe is not the Walkers themselves, but rather the people who are left behind. The pandemic can have a wide variety of effects on the people who are forced to live in such a world, and many turn to madness, attacking others and even taking their own lives to escape the horror.

Often many bands of survivors are thrown together in tense scenarios, leading to larger groups being formed. Once thrown together, these groups are generally kept intact, with inclusion into the group being an exclusive priveledge for those who are deemed trustworthy. This often leads to deaths in groups, as one member can suddenly turn on the rest and work with other groups, or alternatively leave someone they've lived with for months to die.

Since this is the case, trust, companionship and friendship are big themes in The Walking Dead. Difficult moral choices are also regularly featured, forcing characters to decide who lives or dies or other things.